1. Field of the invention
The present invention concerns a method for determining the far-field signature of a plurality of seismic sources.
The invention finds a particularly advantageous application in the field of marine seismic exploration.
2. Description of the Prior Art
Geophysical exploration of the ocean floor is usually carried out by ships towing acoustic sources which emit acoustic waves in different directions towards the ocean floor to be surveyed. The reflected acoustic waves are detected by transducers disposed along cables called streamers also towed by the survey ship. Analysis of the reflected waves relative to the waves emitted by the sources provides data on the structure, nature and composition of the ocean floor.
The acoustic sources used in marine seismics are usually sets of immersed sources or air guns forming a regular one-dimensional or three-dimensional array. To emit an acoustic wave in a given direction emission by the sources is controlled according to a phase law which governs the phase difference between the sources. Specifically, to emit a wave vertically, i.e. perpendicular to the water surface, the nominal phase law is uniform in the sense that the sources all emit at the same time.
To check that the incident acoustic wave is emitted without anomalies and to apply any corrections that may be needed to the signals picked up by the streamer hydrophones a reference signal must be provided each time the air guns are fired and representing the acoustic wave as received at a great distance from its source by the geological structures to be surveyed. This reference signal is the far-field signature FF(t).
In practise, wherever possible the far-field signature of the seismic source system is measured directly by immersing a hydrophone at a depth up to 1 000 m vertically below the array of seismic sources.
In the geophysical exploration of continental plates at a shallow depth of 200 to 300 m, for example, it is not possible to acquire the far-field signature by simple direct measurement. Indirect methods are then used, such as that described in European patent application No 0 400 769. These generally entail deriving a far-field signature FF(t) from the measured near-field signature NFi(t) of each seismic source Si. A hydrophone is associated with each air gun for this purpose and the signal detected by the hydrophone constitutes the near-field signature of the respective source.
Prior art methods which derive the far-field signature FF(t) from the near-field signature NFi(t) apply correction algorithms to each near-field signature and then sum the corrected near-field signatures.
This type of method has the drawback of using theoretical algorithms based on hypotheses which are often somewhat approximate and cast doubt on the accuracy of the result. Also, the processing of the raw data, that is the near-field signatures, requires relatively long computation time on powerful computers.
The technical problem to be solved by the present invention is therefore that of providing a method of determining a far-field signature FF(t) of a plurality of N seismic sources Si (i=1, . . . , N) by measuring a near-field signature NFi(t) of each source Si that is much simpler to implement than known methods and which gives more reliable results.